In the processing of substrates such as semiconductor wafers in the manufacture of circuit chips, and such as substrates for liquid crystal displays (LCDs), the substrates are grouped together in carriers or cassettes for handling by automated equipment. Such substrates are extremely valuable because a great amount of process time and technology has been expended on the processing of the substrates. During processing and between process steps, substrates are conveniently carried in substrate carriers or cassettes which accurately locate the substrates so that they may be handled by automated equipment. However, it is important to determine the exact location of each substrate and determine the presence of a substrate in each intended location. Also, it is important to determine the existence of more than one substrate in a situation where only one substrate is intended to be present.
Previous methods of determining presence and location of substrates has not been altogether satisfactory with the degree of accuracy required in modern processing techniques.